High-temperature variable melting point Sn-Sb lead-free solder pastes using transient liquid-phase powder processing

During this investigation, a high-temperature, variable melting point (VMP) Sn-Sb solder paste was developed. The solder was created by mixing pure Sn and Sb powders together with a flux to form a paste. The proper choice of flux composition and Sn powder size resulted in a Sn-10wt.%Sb solder paste that had an initial melting point of 232 degrees C and solder ball formation at peak temperatures as low as 241 degrees C. This represents a significant reduction in the process temperature that would normally be required for a prealloyed solder with a melting point of 250 degrees C. When this solder paste is reheated, significant remelting does not take place until a temperature above 241 degrees C is reached. In this way, the solder exhibits a VMP. Experiments indicate that this VMP behavior is due to isothermal solidification (or freezing) at the solder temperature owing to the partial transient liquid-phase (TLP) behavior of the solder powder paste.